Why does cloth simulation not work on a NURBS sphere which has been converted to a mesh?
Found the problem:

Select the converted nurbs object, press Tab to go in edit mode, press A to select all elements. Now press Alt+M and select By Distance.

This will remove all the unnecessary vertices, that essentially made the sphere a patchwork become one closed object with no split faces.

The reason Cloth did not work is that it needs a structure, the nurbs sphere was though only patches of separate faces.